The present invention is directed to Adaptive Differential Pulse Code Modulation (ADPCM) encoding, and more particularly to an encoding technique with improved error robustness.
ADPCM encoding has become quite common, and an encoder may have the general configuration wherein a received input signal sample is compared with a predicted value of that signal to produce an error signal which is provided to a quantizer. The quantized error signal is then encoded in a PCM encoder for transmission. Simultaneously, the quantized error signal is fed through an inverse quantizer to recover the error signal which is then supplied to a predictor which generates a prediction of the next signal based upon the most recent history of the signal samples. For an adaptive predictor, the prediction algorithm takes into account the present error signal valve. It is also known to utilize an adaptive quantizer, in which case the error signal is provided to a step size controller which controls the step size used in both the quantizer and inverse quantizer.
Adaptive quantization of a sampled data signal is a well known technique whereby the step size of an n-bit quantizer is varied in order to accommodate a wider dynamic range of the signal source while making most efficient use of the available number of transmission bits. In a system wherein a sample is quantized at each successive sampling instant by n-bits and an adaptive quantizer step size is related to the step size used for quantizing the previous sample, one example of an adaptation algorithm developed by N. S. Jayant, "Adaptive Quantization With One-Word Memory", The Bell System Technical Journal, Vol. 52, No. 7, September 1973, pp. 1119-1144, is EQU .DELTA..sub.i+1 =M(I).DELTA..sub.i, (1)
where i is the time index for the sampling process, .DELTA..sub.i is the quantizer step size and M(I) is a multiplier which depends on the level I at which a sample was quantized. For n quantization bits, I ranges between 1 and 2.sup.n-1.
A disadvantage of this known algorithm is that if, during transmission, an error in the n-bit word has occurred, I' instead of I may be decoded and an incorrect multiplier M'(I) may be used for decoding the next sample. This in turn can lead to an indefinite perpetuation of the decoding error.
In order to overcome these problems of channel bit errors, it has been proposed by Goodman et al "A Robust Adaptive Quantizer", IEEE Trans. Commun. Vol. COM-23, pgs. 1362-1365, November 1975, to add a leakage to the quantizer. Such a leakage can be introduced by modifying equation (1) to: EQU .DELTA..sub.j+1 =.DELTA..sub.j.sup.(1-.alpha.) M(I.sub.j), (2)
where M(I.sub.j) is the step size multiplier which is a function of the quantizer output level number I.sub.j, and (l-.alpha.) is a leakage coefficient provided for the purpose of ensuring decay of the effective channel errors on the step size tracking at the receiving side. .DELTA..sub.j is required to be within a specific interval (.DELTA..sub.max, .DELTA..sub.min), and values of M(I.sub.j) may be as suggested by Jayant such that M(I.sub.j)&gt;1.0 when the amplitude of I.sub.j is in the upper part of the quantizing scale, whereas M(I.sub.j).ltoreq.1.0 when I.sub.j is in the lower part of the quantizing scale.
With such a configuration, if the received quantized level I'.sub.j differs from I.sub.j due to a transmission error, the adaptation of the quantizer step size will then use a wrong multiplier M(I'.sub.j) instead of the correct multiplier M(I.sub.j). The step size offset caused by the channel error will decay exponentially in accordance with the leakage coefficient (l-.alpha.), but the introduction of this leakage will unfortunately result in a decrease in the adaptation speed, which will further result in an increase in the quantization distortion at low and high input signal levels and a decrease in dynamic range.
It would be desirable, therefore, to provide an adaptive quantizer having a high degree of error robustness but which does not suffer from the limited dynamic range and the decreased S/N.